1. Field of the Invention
This invention relates to an apparatus and method for combined thermal and electrical fatigue testing in an operating environment of the reliability of electrical devices. More particularly, this invention provides an apparatus and method that each mimics thermal and electrical stress in a semiconductor during high power switching in an operating environment, such as air. Most particularly, this invention provides and apparatus and method for assessing long term reliability of electronic devices through dynamic testing in an operating environment of semiconductors for voids and short circuits.
2. Discussion of the Related Art
In order to promote, design and realize reliable SiC power devices it is necessary to assess the performance of device components under the influence of their potential operational stress regimes. This is particularly critical for pulsed power device applications, namely, palpitated high power switching, in which the operational environment is dominated by acute cyclic pulsed power actions. The pulsed power action ultimately translates into severe thermal, electrical, and mechanical cyclic stresses in the device materials.
Prior art approaches to assessing the thermal stability for SiC high power devices at high temperatures employ static thermal testing. A usual testing procedure for SiC consists of inducing thermal fatigue by exposing a test structure to a temperature in the range of 300 to 500° C. for more than 100 hours. A typical vacuum furnace, as well known in the art, is employed for such testing. Pre- and post-testing electrical measurements, i.e., current-voltage and specific contact resistance, are compared to obtain an indicator of long-term device reliability. Prior art approaches to assessing pulsed-power thermal stability have consisted of inducing thermal fatigue by exposing the contact-SiC structure to high temperatures (in the range of 900-100° C.) for several minutes in a vacuum environment. A rapid thermal annealer (RTA) is the typical instrument employed for such testing. After exposure, electrical measurements, i.e., current-voltage, and specific contact resistance, are taken and compared to pre-thermally fatigued electrical measurements.
In order to reliably utilize SiC for pulsed power switching applications it is necessary to determine the effects of such cyclic stress regimes on the fundamental pulsed power device components. Another aspect of SiC reliability derives from the high current densities associated with SiC power devices that can cause failure due to electromigration. Electromigration refers to the transport of mass in metals under the influence of current. Electromigration occurs by the transfer of momentum from the electrons to the positive metal ions. When a high current passes through thin metal conductors (metal conductors/interconnects and contacts) metal ions in some regions may pile up whereas voids will form in other regions with resulting respective short-circuits of adjacent conductors and open circuits. Ultimately, electromigration limits device performance and reduces reliability in the long-term.
Prior art test approaches employed for detecting and measuring electromigration are similar to those prior art approaches for static thermal testing. That is, the contact-SiC structure is exposed to high fields in a vacuum for a static non-pulsed duration. Thermal vacuum furnaces do not provide results with respect to thermal stability and electromigration survivability that apply to survivability in air. In other words, a vacuum ambient is not representative of the typical environmental condition these devices typically operate in.
In addition to this shortcoming of prior art static thermal testing, this prior art testing has been directed to measuring individual effects and has not been directed to a combined effects test approach.